Can anybody suggest a solution to make database optimization after migration from Magento 1.9.x to Magento 2.3.4 ?
What kind of optimization you are looking? You can check new relic for your long queries or taking time queries.
You can remove unnecessary attributes. You can disable not used modules. You can optimize queries too.
After migration the database size is around 18GB and it is getting even bigger whenever i reindex. Also reindexing process takes around 72 hours to complete. I have 1 store with 1.6 Million products.
You can put indexs on update on schedule and set a cron. Apart from that check sleep queries, if sleep queires are running you should kill all and find out what is the cause of sleep queires.